Add flag to db.backend indicating whether JOINs are supported
- Used to disable tests which contain JOINs in their queries

Conflicts:
	django/db/backends/__init__.py
Modify contrib.auth queries and model ordering to work with DBs that do not support JOINs

Conflicts:
	django/contrib/auth/management/__init__.py
Fix view_on_site url pattern to match MongoDB ObjectIds

Conflicts:
	django/contrib/admin/sites.py